Some excavators, for example the model P&H 2800 excavators manufactured by HARSNISCHFEGER CORP. of Milwaukee, Wis., U.S.A. have a bucket mounted at the end of a boom. The bucket has an open front side and a large door on its rear side. The lower edge of the front side has teeth for digging into earth. Excavators of this type are typically very large and are used in open pit mining and the like where vast quantities of earth must be moved. The buckets of such excavators are typically on the order of 3 meters high by 3 meters wide by 3 meters deep and typically have a capacity of 15 to 50 cubic meters of earth.
In a typical mining operation an excavator fills up large dump trucks with coal or ore-containing earth. The trucks haul the earth away for further processing. The excavator operator first scoops up earth with the door on the bucket closed and then moves the boom so that the full bucket is above a truck to be filled, with the door on the rear side of the bucket facing downward. The operator then opens a latch which holds the door shut. The door falls open under its own weight and the weight of the earth in the bucket. The earth in the bucket then falls into the truck. The door closes and latches as the operator moves the bucket into position to scoop up another load of earth.
A problem excavators of this type is that the bucket doors take extreme punishment. The doors slam shut very hard. In some cases the doors slam shut so hard that they bounce open before the latch can engage properly. Over time, even though they are typically fabricated from steel plate which is between 2.5 and 5 centimeters thick and are reinforced by steel ribs, these doors develop cracks. When a door gets too severely cracked it must be taken out of service and repaired.
Most excavators are equipped with a snubber to reduce damage to the door by reducing the impact with which the door closes. Snubbers are subjected to extreme forces. Excavator bucket doors are rapidly forced open by the weight of several tonnes of earth and then swing closed again. The snubbers operate in dirty conditions. In many mine sites excavators operate virtually continuously all day and all night for weeks on end.
One type of snubber, which is described in Australian patent application No. 8780908 uses friction to damp the motion of a bucket door. A typical friction snubber comprises a stack of friction discs. Friction disk type snubbers may be obtained, for example, from ESCO of Portland Oreg., U.S.A. or HARSNISCHFEGER CORP. of Milwaukee, Wis. A first set of the discs is mounted to the excavator bucket. A second set of the discs is interleaved between the discs in the first set. the second set of discs is mounted to a linkage connected to the bucket door. When the door opens or closes the second set of discs turns with respect to the first set of discs. The friction discs are all clamped tightly together with a bolt and a spring washer. Friction between the discs damps the opening and closing of the door.
One problem with friction type snubbers are that the friction discs wear as the door opens and closes. This wear is made worse by dirt and moisture which can get between the disks. To keep friction snubbers working effectively the bolt which holds the friction discs together must be periodically tightened. In many cases, these bolts are not tightened properly or often enough and door damage results. Another problem with friction type snubbers is that they damp opening of the door as well as closing of the door.
A second type of snubber comprises a thick strip of elastomer which is connected to the door by a linkage. When the door closes the elastomer strip stretches. A problem with elastomer strip snubbers is that the elastomer eventually wears out and ceases to damp the closing of the door. The elastomer bands also tend to break frequently when the weather is cold.